/R
width mm / profile % R rim inch
/R
read it from the tire wall, e.g. 225/45 R17
Popular sizes (sets the new tire)
km/h
Both wheels at the same road speed
Compare your current tires with a new size. See the diameter, sidewall and circumference side by side, how far off your speedometer will read, and which other sizes fit the same wheel well.
| Current | New | Difference |
|---|
| Size | Diameter | Diff. | Width |
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A tire size like 225/45 R17 means 225 mm section width, a sidewall 45% as tall as the width, on a 17-inch rim. Everything else follows from those three numbers.
Keep the new diameter within about ±3% of the original (many tire makers and inspectors use ±2%). A bigger diameter makes the speedometer read low, which is not allowed in the EU (UNECE R39: the speedometer must never show less than the real speed). Also check the load index, speed rating and that the wider tire clears the arches and suspension at full lock.
This calculator compares two tire sizes and shows what changes when you fit new wheels or tires. From size codes like 205/55 R16 and 225/45 R17 it works out sidewall height, overall diameter, circumference, revolutions per km or mile, the percentage difference, the true speed when the speedometer shows a given value, and the change in ride height. Drivers, tire shops, mechanics and car and motorcycle enthusiasts use it to check a plus-size upgrade, a winter set or an off-road tire before buying.
225/45 R17 is a good plus-one replacement for 205/55 R16: only 0.38 % larger, so the speedometer reads just 0.4 km/h low at 100 km/h. Most factory speedometers are set to read a few percent high, which absorbs this, but check the 20 mm extra width for clearance at full steering lock.
225 is the section width in millimeters. 45 is the aspect ratio: the sidewall is 45 % as tall as the width, so 101.25 mm. R means radial construction, and 17 is the rim diameter in inches. A full marking such as 225/45 R17 94W adds the load index (94 = 670 kg or 1477 lb per tire) and the speed rating (W = 270 km/h or 168 mph). These codes follow the ETRTO and ISO conventions used worldwide on passenger tires.
Diameter = rim diameter × 25.4 + 2 × width × aspect ratio / 100, in millimeters. For 225/45 R17: 17 × 25.4 = 431.8 mm for the rim, plus two sidewalls of 225 × 0.45 = 101.25 mm each, gives 634.3 mm, or 24.97 in. Circumference is π × diameter, here 1992.7 mm, and revolutions per km = 1 000 000 / circumference = 502. The real rolling diameter under load is about 2–3 % smaller than this nominal figure.
Yes. The speedometer counts wheel or gearbox revolutions, so a tire with a larger diameter covers more distance per turn than the speedometer assumes, and it reads low. True speed = indicated speed × new diameter / old diameter. A tire 3 % larger means 100 km/h on the dial is really 103 km/h, and 60 mph is really 61.8 mph. The odometer under-counts by the same 3 %. In the EU, UNECE R39 requires that a speedometer never show less than the true speed.
The common guideline is to keep the overall diameter within ±3 % of the original size, and many tire makers, shops and inspection bodies use a stricter ±2 %, or allow only a smaller deviation on the larger side. Staying close keeps the speedometer, ABS, stability control and gearing working as designed. Also match or exceed the original load index and speed rating, and check that a wider tire clears the arches, suspension and brake parts at full lock and full compression. Local registration rules decide what is legal.
Plus sizing means fitting a larger rim while keeping the overall diameter about the same, by using a lower-profile tire. Plus one adds 1 inch of rim and usually 10 mm of width with 5 to 10 points less aspect ratio, for example 205/55 R16 to 225/45 R17, which changes the diameter by only 0.4 %. The shorter, stiffer sidewall sharpens steering and adds grip, but rides firmer, is easier to damage on potholes and usually costs more.
Yes, by half of the diameter increase, because only the distance from the axle center to the ground changes. Ride height change = (new diameter − old diameter) / 2. Going from 265/70 R17 (802.8 mm, 31.6 in) to a 33 in tire (838.2 mm) lifts the axles and body by about 17.7 mm, or 0.7 in. Suspension lift kits raise the body only, while bigger tires also lift the axle and differential, which is what improves off-road clearance.